Traditionally, bats for baseball, softball and like games have been made of wood, and the handling qualities, impact resistance, and sound of wood bats are recognized as desirable. The wood structure also dampens the reverberating vibrations in the bat that results from a hit. This dampening prevents stinging of the batter's hands, also a desirable quality.
In recent years, many softball bats have been made as welded shells of aluminum. Aluminum bats drive the ball farther than wood bats, but give an objectionable ringing sound when they contact the ball. Further, they sometimes break along their weld line; and dents cause stress concentrations which induce failures.
A type of bat designed to create the characteristics of a wood bat was disclosed in U.S. Pat. No. 4,032,143, and an improved version is shown in said co-pending application Ser. No. 06/181,175. This bat is of composite construction utilizing an aluminum core tube and a foamed polyurethane body of the type that forms an integral skin. This bat has proved to be comparable to a wood bat in many respects, in that it survives abuse much like a wood bat and sounds like one when it strikes the ball. However, the polyurethane foam was found not to add in any substantial way to the stiffness of the bat; the deflection of the core tube alone and the completed bat are approximately the same. Therefore, in order to obtain the necessary stiffness the tube had to be of such weight that the density of the polyurethane foam had to be made less than was desirable to remain within weight restrictions.